A liquid crystal display (LCD) panel is a device that uses birefringent liquid crystal molecules, the alignment of which is controlled to allow light to pass through or to shield light (ON/OFF of the display). There are several modes of liquid crystal alignment of LCDs, including: twisted nematic (TN) mode that is characterized in that liquid crystal molecules with positive dielectric constant anisotropy are twisted 90° when viewed in the normal direction of a substrate; vertical alignment (VA) mode that is characterized in that liquid crystal molecules with negative dielectric constant anisotropy are aligned perpendicularly to the substrate surface; and in-plane switching (IPS) mode and fringe field switching (FFS) mode that are characterized in that liquid crystal molecules with either positive or negative dielectric constant anisotropy are aligned horizontally to the substrate surface, and a transverse electric field is applied to the liquid crystal layer.
A widespread system to drive a LCD panel is an active matrix driving system. This system provides high-quality images, which is attributed to active elements, including a thin film transistor (TFT), disposed in each pixel. As examples of substrates for LCD panels provided with TFTs, mention may be made of an active matrix substrate in which a plurality of scan signal lines and a plurality of data signal lines cross each other, and at each of the intersections of these lines, a TFT and a pixel electrode are provided. A common type of LCD panel is provided with a common electrode in the active matrix substrate or the counter substrate such that a voltage is applied to the liquid crystal layer thorough this pair of electrodes.
In such an active matrix substrate, display defects may occur when a short circuit occurs between lines of different kinds. One possible solution for such a trouble is laser repair treatment that involves laser radiation for cutting, for example, at least one of a drain line and a source line of a target TFT, thereby preventing transmission of a scan signal and an image signal to a pixel electrode (for example, Patent Literature 1).
FIG. 11 is a plan view schematically showing one example of a conventional laser repair treatment. In the example shown in FIG. 11, a pixel electrode PX and a counter electrode CT are provided for each of regions defined by gate lines GL and drain lines DL, and each drain line DL is electrically connected to the corresponding pixel electrode PX through a TFT and a source line SL. The TFT is composed of parts of the gate line GL, the drain line DL and the source line SL. More specifically, the drain line DL includes a trifurcated end, and the source line SL extends into the spaces between the three branches of the drain line DL. The two branches of the source line SL in the spaces are integrated into one, and the integrated source line SL extends to overlap the pixel electrode PX, and is electrically connected to the pixel electrode PX via a contact hole H.
Patent Literature 1 teaches that when there is an unwanted object that overlaps a TFT, the source line SL is a target of laser repair. The laser radiation targeted to the source line SL cuts the source line SL so that the pixel electrode PX and the drain line DL connected to the TFT with the unwanted object are electrically separated from each other. Another strategy is laser radiation to the contact hole H. This makes a short circuit between the pixel electrode PX and the counter electrode CT, so that the pixel electrode PX and the counter electrode CT have the same potential. Consequently, the pixel becomes a black spot.